Drilling mud degassers for oil wells



' March 22, 1966 P. H. GRIFFIN Ill, ETAL DRILLING MUD DEGASSERS FOR OILWELLS Filed Sept. 11, 1963 4 Sheets-Sheet 1 PHIL H. GRIFFIN,||I JOHN w.MELTON,JR..

z W y March 22, 1966 P. H. GRIFFIN 1 11, ETAL 3,241,295

DRILLING MUD DEGASSERS FOR OIL WELLS Filed Sept. 11, 1963 4 Sheets-Sheet2 PHIL H. GRIFFIN, III

JOHN W. MELTON, UR.

P. H. GRIFFIN m, EI'AL 3,241,295

DRILLING MUD DEGASSERS FOR OIL WELLS 4 Sheets-Sheet 5 //1Ve/7/b/1s.-

PHIL H GRIFFIN m JOHN W. MELTON, JR.

March 22, 1966 Filed Sept. 11 1963 March 22, 1966 P. H. GRIFFIN m, ETAL3,241,295

DRILLING MUD DEGASSERS FOR OIL WELLS Filed Sept. 11 1963 4 Sheets-Sheet4 INVENTORS:

PHIL H. GRIFFIN, III

JAKE MELTON, JR. By: W F%' W Km AT'T'Y.

IOO

United States Patent 3,241,295 DRILLING MUD DEGASSERS FOR OIL WELLS PhilH. Griflin III, Fort Worth, Tex., and John W. Melton, Jr., Lafayette,La., assignors to Bass Brothers Enterprises, Inc., Fort Worth, Tex.

Filed Sept. 11, 1963, Ser. No. 308,314 8 Claims. (Cl. 55-165) Thisapplication is a continuation-in-part of our earlier application SerialNo. 37,685, filed June 21, 1960, and application Serial No. 299,317,filed August 1, 1963, both now abandoned.

This invention relates to new and useful improvements in drilling muddegassers for oil wells of the general type disclosed, for example, inUS. Patents No. 2,748,884 dated June 5, 1956, and No. 2,869,673 datedJanuary 20, 1959, to R. W. Erwin.

The degassers disclosed in those patents are mounted above the level ofmud in the mud trough, and an object of the present invention is tofacilitate positioning of the degasser either beside or below as well asabove the mud level, as is best suited for particular operatingconditions.

A feature of the invention resides in the provision of means comprisingan automatically actuated valve for controlling the level of mud in thedegasser tank, so that even if the tank is disposed at or below thelevel of mud in the trough, it is positively safeguarded againstflooding.

Another feature of the invention involves a novel structural arrangementof the degasser tank per se, the same being vertically rather thanhorizontally disposed and containing a set of vertically spaced bafllesover which the mud flows in a casading manner to facilitate and expeditedegassing thereof.

Another feature of the invention, contemplated by a modified embodimentthereof, resides in the provision of a vacuum actuated pump of a novelconstruction, forcausing the flow of mud through the degasser tank, asdistinguished from the jet type propelling means disclosed in theaforementioned patents.

Another feature of the invention, contemplated by another modifiedembodiment, facilitates the use of a centrifugal pump for causing theflow of mud through the degasser tank, with valve means for effectivelycontrolling the mud flow in relation to the mud level in the tank.

Wiih the foregoing object and features in view and such other objectsand features as may become apparent as this specification proceeds, theinvention will be understood from the following description taken inconjunction with the accompanying drawings, wherein like characters ofreference are used to, designate life parts, and wherein:

FIGURE 1 is an elevational view of the degasser apparatus, with certainportions thereof shown in section;

FIGURE 2 is a vertical sectional view of the degasser tank;

FIGURE 3 is an elevational view, similar to that shown in FIGURE 1 buton a reduced scale and showing a modified embodiment of the invention;

FIGURE 4 is an elevational view, partly in section, of the pump used inthe embodiment of FIGURE 3;

FIGURE 5 is a bottom plan view of the pump;

FIGURE 6 is a fragmentary sectional view on an enlarged scale, takensubstantially in the plane of the line 6-6 in FIGURE 4;

FIGURE 7 is a view, partly in vertical section and partly in elevation,showing another modified embodiment; and

FIGURE 8 is a diagramatic view showing connections of a valve used inthe embodiment of FIGURES 4-6.

Referring now to the accompanying drawings in detail, more particularlyto FIGURES l and 2, the mud deice gasser apparatus in accordance withthe invention is designated generally by the reference numeral 10 and,principally, embodies in its construction a de-gasser tank 11 which maybe positioned above, below, or, as shown, at the level 12 of mud in atank 13 into which the mud is discharged from the well through a pipe14, the same mud level 12 also existing in a tank 15 from which degassedmud is subsequently pumped or otherwise delivered through a pipe 16 backto the well.

The degasser tank 11 comprises a cylindrical, vertically disposed bodymounted by suitable supporting brackets 17 on a platform or skid 18 andprovided with a removable cover 11a to facilitate access to the interiorthereof for purposes of inspection or repair. A tubular adapter 19extends into the tank through the cover 11a and is connected to an inletpipe 20 which communicates with the tank 13 as shown. A butterfly valve21 is provided in the adapter 19 for actuation by an arm 22 as will behereinafter described, the purpose of this valve being to regulate thevolume of mud passing into the degasser tank 11 from the tank 13,whereby to prevent. the degasser tank from flooding.

The lower end of the tank is provided with an adapter 23 connected to adischarge pipe 24 which communicates with the tank 15, the pipe 24 beingshown as provided, as in the aforementioned Erwin patents, with a jet 25for propelling the degassed rnud therethrough and simultaneouslycausingmud to be drawn into the trough and from the tank 13 through theinlet pipe 20 into the tank 11, as set forth in the aforesaid patents.The lower end of the pipe 24 may be equipped with a wear plate 26.

The interior of the degasser tank 11 is shown as provided with a set ofvertically spaced bafiles, namely, a conical upper baflle 27, aninverted frusto-conical inter mediate baflle 28, and a conical lowerbaflle 29. The upper baffle 27 is equipped with an annular rim 30 whichis spaced outwardly from the marginal edge ofthe baflle by a set ofsupporting straps 31, so that mud flowing downwardly through the adapter19 may, after becoming deposited on and evenly distributed over theb-aflle 27, cascade downwardly as indicated at 32 onto the marginal edgeportion of the baflle 28.

The marginal edge of the baffle 28 is provided with an upstanding flangeor rim 33, within which there is provided an apertured pipe or conduit34 whereby water under pressure may be discharged onto the baflle 28 forpurposes of washing the interior of the tank, or whereby suitablechemicals may be introduced for mixing with the mud, if so desire-d. Thewater or chemicals are delivered to the conduit 34 from a suitablesource exteriorly of the tank by a suitable pipe (not shown). The battle28 is provided with a central opening 35 whereby mud may flow onto thelower bafile 29, for subsequent accumulation in the bottom of the tank.

It is to be noted that the slope of the intermediate baffle 28 isgreater than that of the upper baflle 27 and that the slope of the lowerbaffle 29 is also greater than that of the baflle 28. Thus, in flowingdownwardly from the adapter 19, the mud first is evenly spread over theupper baffle 27, then re-directed toward the center of the tank by theintermediate baflle 28, and then again spread evenly over the lowerbaffle 29, until it drops to the bottom of the tank. The tank bottom,also of inverted conical form, safeguards against sediments dropping outof the mud, and the different slopes of the several baflles, as abovenoted, accelerate the flow of mud during its cascading over the baflies,whereby a self-cleaning action of the baffies is produced.

The pressure within the degasser tank 11 is normally maintained at lessthan atmospheric pressure in order that gas bubbles entrained in the mudpassing through the tank will expand and break away from the mud. Thesubatmospheric pressure within the tank is produced not only by the jetdischarge 25 of the mud from the tank, but also and, mainly by asuitable pump 36, mounted on the platform or skid 18, and driven by asuitable motor 37. The gases released from the mud in the tank 11 enterthe open, lower end of a tube 38 which extends from the underside of thebaffle 29, through the baflles 29, 28, 27 and through the tank cover 11ato a conventional threeway valve 39 mounted by a bracket 40 on top ofthe cover 11a. The valve 39 also has a port 41 communieating with theatmosphere and a suction line or pipe 42 extending to a conventionalfloat-actuated liquid trap 43 mounted on the skid 18. The trap 43communicates through a pipe 44 with the pump 36, while the exhaust ofthe.pump to the atmosphere is indicated at 45. If de-. sired, a shut-offvalve 46, a sight glass 47 and a pressure gauge 48 may be provided inthe line 42, as shown.

Sub-atmospheric pressure within the tank 11 is regulated by the valve 39in response to raising and falling of a float 49 on the mud level 50 inthe bottom of the tank 11. The float 49 is secured to the lower end of arod 50 which extends slidably through the batfles 29, 28, 27 and througha packing gland 51 in the tank cover 11a, being operatively connected atits upper end as at 52 to an actuating arm 53 of the valve 39. As themud level 50 in the tank rises, the float 49 causes the three-way valve39 to open the port 41 to the atmosphere while at the same time cuttingoff communication through the line 42 to the pump 36, thus allowingatmospheric pressure to enter the tank and reduce the vacuum therein. Onthe other hand, when the mud level falls, the valve port 41 is closed,and a greater sub-atmospheric pressure is produced in the tank bysuction of the pump 36 through the lines 44, 42 and 38.

The aforementioned butterfly valve 21 in the adapter 19 is opened andclosed automatically in response to air pressure within the tank 11,that is, in response to variations in pressure of air within the tank,this being effected by the provision of a vacuum cylinder 54 which ispivotally mounted as at 55 on the tank cover 11a and includes a piston56 with a piston rod 57 pivoted as at 58 to the valve actuating arm 22.The cylinder 54 is connected by a vacuum line 59 to the tube 38 and asthe pressure within the tube 38 and tank 11 decreases, the piston 56 isretracted into the cylinder 54 and causes the valve 21 to open, thuspermitting more mud to enter the tank. On the other hand, when pressurewithin the tank increases, outward sliding of the piston 56 in thecylinder 54 causes the valve 21 to close, thus restricting the flow ofmud into the tank accordingly.

Means are provided for biasing the valve 21 to its closed position, suchmeans comprising a cylinder 59', pivotally mounted as at 60 on the tankcover 11a adjacent the cylinder 54, and having a reciprooable piston 61,springbiased by a compression spring 62, with a piston rod 63 extendingfrom the piston 61 and pivoted to the arm 22 as at 64.

In order to prevent degassed mud from flowing back to the tank 11, asuitable check valve 65 is provided in the discharge pipe 24 between theadapter 23 and the jet 25.

When the apparatus is in operation, the rate of flow of mud therethroughis controlled by the action of the jet 25, regulated by a suitable valve66, whereby the rate of mud flow out of the tank and the mud level inthe bottom of the tank are maintained substantially constant. How ever,if the inflow of mud should become greater than the outflow, rising ofthe mud level in the tank will cause the float 49 to vent the valve 39to the atmosphere, and the resultingly increased pressure of air in thetank and in the lines 38, 59 will cause the piston 56 in the cylinder 54to close the valve 21, thus reducing or shutting off the inflow of mudthrough the adapter 19. Conversely, with the inlet of mud reduced orstopped, lowering of the mud 4 level 50 in the tank will produce agreater vacuum in the tank, thus causing the cylinder 54 to open thevalve 21 to permit a greater inflow of mud into the tank. As a combinedresult of these automatic operations, both the mud level andsub-atmospheric pressure within the tank will be maintainedsubstantially constant.

The purpose of the trap 43 is to prevent liquids from entering thevacuum pump 36, the trap fulfilling this function by collecting andstoring the undesired liquid which may be occasionally drained from thetrap through a drain valve 99. The latter is manually operated andinstances may arise when draining of the trap is overlooked, in whichevent an excessive amount of liquid accumulated in the trap will causethe float in the trap to close a valve on the line 42, thus preventingfurther vacuum to form in the degasser tank 11. In order to eliminatethis difficulty and to provide for automatic draining of the trap 43, acheck valve 100 is connected to the trap 43 so that it communicates withthe trap even when the drain valve 99 is closed, and a line 101 extendsfrom the check valve 100 to the interior of the degasser tank, as shown.The valve 100 permits liquid to flow from the trap 43 to the tank 11only. When the pump 36 is stopped, the vacuum in the line 42 will leakoff more rapidly than the vacuum in the tank 11 and when this conditionarises, the liquid accumulated in the trap will flow through the checkvalve 100 and line 101 into the tank 11, thus automatically draining thetrap.

Referring now to FIGURES 3-6 which show a modified embodiment of theinvention denoted generally by the reference numeral 70, the same isadapted to be mounted above the level of mud in a mud trough 71 havinginlet and outlet tanks or sumps at opposite ends thereof correspondingto the tanks 13 and 15 of FIGURE 1, with the degasser tank 11 beingarranged substantially as described in connection with the embodiment11).

However, in place of the jet 25 for propelling the mud through thedegasser tank, a vacuum pump designated generally by the numeral 72 isemployed. This pump is mounted above the mud trough 71 adjacent the tank11 and comprises a suitable housing 73 having a partition 74 thereinwhich separates the interior of the housing into a pair of chambers 75,76. The bottom of the housing 73 is provided with a pair of dooropenings having a pair of doors 77, 78 in communication with therespective chambers 75, 76, the doors 77, 78 being hingedly mounted asindicated at 79 so that they are normally urged by gravity to theirclosed position. A manifold 80 is provided at the underside of thehousing 73 in communication with the doors 77, 78 and is connected by aduct or pipe 24a to the mud outlet of the degasser tank 11.

The housing 73 also has a front wall provided with openings for a pairof doors 81, 82 in communication with the respective chambers 75, 76,these doors being hingedly mounted as at 83 so that they are normallyurged by gravity to their closed position.

In this embodiment, the aforementioned motor 37, the vacuum pump 36 andthe liquid trap 43 are mounted on top of the housing 73 and the suctionline 42 which extends between the valve 39 on the tank 11 and the liquidtrap 43 is provided with a T 84 having a branch suction line 85connected thereto. The suction line 85 communicates with one port of afour-way valve 86, also mounted on top of the housing 73, while theexhaust line 87 of the pump 36 communicates with another port of thisvalve. The two remaining ports of the four-way valve 86 have conduits88, 89 connected thereto for communication with the respective chambers75, 76, it being understood that in one position of the valve 86, airmay be withdrawn from the chamber 75 through the conduit 88 and lines85, 42, 44 while air under pressure from the pump 36 is delivered intothe chamber 76 through the lines 87 and 89. Conversely, in a secondposition of the valve 86, air may be withdrawn from the chamber 76 anddelivered under pressure into the chamber 75.

The valve 86 is actuated by an arm 90 which has operatively connectedthereto as at 91 the upper end of a rod 92 which extends into a cylinder93. The latter is provided at the outside of the housing 73 andcommunicates with the chamber 75 by a pair of connecting passages. 94,95. A mud level responsive float 96 is slidable in the cylinder 93 onthe rod 92 and when the float rises sufficiently high in the cylinder,it comes in abutment with an adjustable stop 97 on the rod 92, whereuponfurther upward movement of the float causes the rod 92 to slide upwardlyto actuate the valve 86 through the medium of the arm 91 In operation,mud under sub-atmospheric pressure discharged from the degasser tank 11stands in the pipe 24a and manifold 80, while the pump 36 exhausts airfrom the chamber 75 through the valve 86 and conduit 88. When the vacuumin the chamber 75 is greater than that in the manifold 80, the door 77is caused to open, with the result that the mud enters through the opendoor into the chamber 75. With the rising of the mud in the chamber 75and in the cylinder 93 communicating therewith, upward movement of thefloat 96 eventually reverses the valve 86, so that air under pressurefrom the pump 36 is delivered into that chamber, As this occurs, thedoor 77 is automatically closed and the mud in the chamber 75 flows bygravity through the door 81 into the discharge sump of the mud trough71. With the discharge of mud from the chamber 75 and cylinder 93,lowering of the float 96 causes the same to engage another stop 98 onthe lower portion of the rod 92, thus sliding the rod downwardly andreturning the valve 86 to its position where suction is again applied tothe chamber 75 for the next cycle of operation.

The pumping operation in the chamber 76 is reverse to that in thechamber 75, that is, while mud is being discharged from the chamber 75through the door 31, mud is being drawn into the chamber 76 through thedoor 78, and conversely, when mud is drawn into the chamber 75 throughthe door 77, mud is discharged from the chamber 76 through the door 82.As a result, the flow of mud through the pump 72 is substantiallycontinuous by virtue of the two, reversely acting pump chambers.

Referring now to FIGURE 7, the same illustrates another modifiedembodiment of the invention including a degasser tank 111 through whichmud is propelled by a centrifugal pump 112 connected to the mud outlet24 of the tank, as by a pipe or conduit 113. To the extent thatconstruction and arrangement of the tank 111 is similar to the degassertank 11, corresponding reference numerals are used in the illustrationof FIGURE 7.

The float 49 on the mud level 50 in the tank 11 is carried by avertically swingable arm 114 which operates the three-way valve 115mounted exteriorly on the side of the tank. The arm 114 extends from thevalve into the tank through an opening 116 in the tank Wall, suchopening being covered by the valve 115 to prevent leakage of airtherethrough. The valve 115 is connected to the suction pump by asuction line 42, has a port 41 vented to the atmosphere, and a suctionpipe 38 which communicates with the interior of the degasser tank. Theoperation of the float-controlled valve 115 is the same as that of theaforementioned valve 39, but it is to be noted that since the valve 115is mounted at the side of the tank rather than on top of the tank cover11a, it is not necessary to pass the float arm 114 and the suction pipe38 through the various baflles 27, 28, 29, as in the embodiment of thetank 11.

A pressure-responsive governor 117 of a conventional, commercial type ismounted as by a bracket 118 on the tank cover 11a and is operativelyconnected as by linkage 119 to the arm 22 of the mud inlet valve 21 atthe top of the tank. The governor 117 has a pilot control 120 mounted atthe side of the tank 111 and connected by a line 121 to the vacuum pumpor to the line 42, as desired. The connection between the governor 117and the control is by a line 122, and a vacuum line 123 extends from thecontrol 120 to the interior of the tank.

When the mud level in the tank falls and the air pressure in the tank isdecreased by the valve 115, the air pressure change is sensed by thepilot control 120 through the line 123 and the control responds bycommunicating the line 122 with the suction line 121 so that thegovernor 117 opens the valve 21 and admits more mud into the tank.Conversely, when the mud level in the tank rises, the governor closesthe valve 21 to decrease or stop the inflow of mud into the tank.

A similar governor 124, connected by a line 125 to the control 120, maybe used to operate a valve 126 in the discharge 127 of the pump 112 bywhich the mud is drawn through the tank 111 and delivered to the mudsump. The arrangement is such that when the mud level in the degassertank falls, the regulator 124 closes the discharge valve 126, thuspermitting the mud in the tank to rise to a higher level while the mudinlet valve 21 is opened by the regulator 117. On the other hand, if themud level rises, the regulator 124 will open the discharge valve 126,thus permitting excess mud to be Withdrawn from the tank by the pump 112while the inflow of mud into the tank is decreased or shut off byclosing of the valve 21 by the regulator 117.

The mud valve 126 may be located downstream from the pump 112 as shown,or between the pump 112 and the tank outlet 24, or in a by-pass aroundthe pump, as dis closed in the aforementioned application filed August1, 1963.

While in the foregoing there have been shown and described the preferredembodiments of the invention, various modifications may become apparentto those skilled in the art to Which the invention relates. Accordingly,it is not desired to limit the invention to this disclosure and variousmodifications and equivalents may be resorted to, falling within thespirit and scope of the invention as claimed.

What is claimed as new is:

1. In a mud degasser apparatus, the combination of a mud degasser vacuumtank having .a mud inlet and a mud outlet for continuous flow of mudthrough the tank, means for maintaining the interior of said tank atsub-atmospheric pressure, valve means provided in said mud inlet forcontrolling the rate of flow of mud into the tank, means responsive tovariations of level of mud in said tank for varying the sub-atmosphericpressure in the tank, and means responsive to variations of pressure inthe tank for opening and closing said valve means.

2. In a mud degasser apparatus, the combination of a mud degasser vacuumtank having a mud inlet and a mud outlet for continuous flow of mudthrough the tank, means for maintaining the interior of said tank atsub-atmospheric pressure, valve means provided in said mud inlet forcontrolling the rate of flow of mud into the tank, means responsive tovariations of level of mud in said tank for varying the sub-atmosphericpressure in the tank, and means responsive to variation of pressure inthe tank for opening and closing said valve means, said pressureresponsive means comprising a cylinder having piston means operabletherein for actuating said valve means and having a conduit at one endthereof connecting with the interior of said tank for actuating saidpiston means.

3. In a mud degasser apparatus, a mud degasser vacuum tank having a mudinlet at the top and a mud outlet at the bottom thereof, a plurality ofvertically spaced cascade baflles provided in said tank in the path ofmud flowing from said inlet to said outlet, the slope of the bafflesbeing progressively steep in a downwardly direction to render the sameself-cleaning, a vacuum pump, a suction line extending from said pump tothe interior of said tank, a valve provided in said suction line forselectively communicating the interior of the tank with said pump andwith the atmosphere, float means responsive to variations of mud levelin the bottom of the tank for actuating said valve, valve means providedin the inlet of said tank for controlling the rate of flow of mudthereinto, and means responsive to actuation of said valve for actuatingsaid valve means.

4. In a mud degasser apparatus, the combination of a mud degasser vacuumtank disposed in a vertical position and having a mud inlet and a mudoutlet respectively at the top and bottom thereof for continuous flow ofmud through the tank, a vacuum pump, a suction line extending from saidpump to the interior of said tank for maintaining the same atsub-atmospheric pressure, a valve provided in said suction line forselectively communicating the interior of the tank with said pump andwith the atmosphere, float means responsive to variations of mud levelin the bottom of the tank for actuating said valve whereby thesub-atmospheric pressure in the tank may be maintained substantiallyconstant, valve means provided in the inlet of the tank for controllingthe rate of flow of mud thereinto, and means responsive to variations inthe sub-atmospheric pressure in the tank for actuating said valve means,whereby the level of mud in the bottom of the tank may be maintainedsubstantially constant.

5. The device as defined in claim 4 wherein said last mentioned meansinclude a vacuum cylinder in communication with the interior of saidtank, said cylinder having a reciprocable piston rod operativelyconnected to said valve means.

6. In a mud degasser apparatus, the combination of a vacuum tank havinga mud inlet and a mud outlet, for continuous flow of mud through thetank means for maintaining air pressure in said tank below that of theatmosphere, valve means provided in said mud inlet for controlling therate of flow of mud into the tank, means responsive to raising of themud level in the tank for increasing the air pressure in the tank andconversely for de creasing the air pressure in response to lowering ofthe mud level, and means responsive to increasing and decreasing of theair pressure in the tank for respectively closing and opening said valvemeans.

7. In a mud degasser aparatus, the combination of a vertically disposedvacuum tank having a mud inlet and a mud outlet respectively at the topand bottom thereof for continuous flow of mud through the tank, a gassuction line extending outwardly from said tank, a vacuum pump, athree-way valve for communicating said suction line selectively withsaid vacuum pump and with the atmosphere to respectively decrease andincrease the air pressure in said tank, valve means provided in said mudinlet for controlling the rate of flow of mud into the tank, a floatprovided in the tank and operatively connected to said three-way valvewhereby to respectively increase and decrease the air pressure in thetank when the mud level in the tank rises and falls, and a fluidoperator connected to said suction line and to said valve means forrespectively closing and opening the valve means in response to anincrease and decrease of air pressure in said tank.

8. A mud degasser vacuum tank having a mud inlet and a mud outlet forcontinuous flow of mud through the tank, means for maintaining theinterior of said tank at sub-atmospheric pressure, valve means providedin said mud inlet for controlling the rate of flow of mud into the tank,means responsive to variations of level of mud in the tank for varyingthe subatmospheric pressure therein and means responsive to variationsin subatmospheric pressure for actuating said valve means.

References Cited by the Examiner UNITED STATES PATENTS 1,268,594 6/1918MacKenzie 103-236 1,518,890 12/1924 Aikman 103-236 1,556,098 10/1925Gibson 55-39 1,708,471 4/1929 Farnsworth 103-246 1,710,178 4/1929McMurray 55-206 2,015,127 9/1935 Rieger 103-236 2,126,596 8/1938 Wood55-193 2,195,898 4/1940 Newton 55-190 2,748,884 6/1956 Erwin 55-1932,815,765 12/1957 Adelson 137-205 2,869,673 1/1959 Erwin 55-192 REUBENFRIEDMAN, Primary Examiner.

1. IN A MUD DEGASSER APPARATUS, THE COMBINATION OF A MUD DEGASSER VACUUMTANK HAVING A MUD INLET AND A MUD OUTLET FOR CONTINUOUS FLOW OF MUDTHROUGH THE TANK, MEANS FOR MAINTAINING THE INTERIOR OF SAID TANK ATSUB- ATMOSPHERIC PRESSURE, VALVE MEANS PROVIDED IN SAID MUD INLET FORCONTROLLING THE RATE OF FLOW OF MUD INTO THE TANK,